Add links to README (and few tweaks)

README.md

@@ -12,7 +12,10 @@ at Python developers anyway.
 
 The primary target are plugins for Icinga 2.
 
-Documentation of plugin API is at TODO and TODO.
+The plugin API is documented at
+[monitoring-plugins.org](https://www.monitoring-plugins.org/doc/guidelines.html)
+and in [Icinga 2
+documentation](https://icinga.com/docs/icinga-2/latest/doc/05-service-monitoring/#plugin-api).
 
 Project goals (and non-goals)
 ----
@@ -38,7 +41,11 @@ Please send anything you have on your mind to
 
 I currently want to know if anyone else has tried to use the code :-)
 
-(For patches you can use `git send-email` (TODO link to Drew DeVault's site))
+For patches you can use `git send-email`. For more info, see
+[git-send-email.io](https://git-send-email.io/) (or put "git send-email" into
+your favourite search engine).
+
+(Emails sent there are currently not public, because I'm lazy. If any
 
 Licence
 ----